1. Field of the Invention
The present invention relates to a focus detection technique in an image capturing apparatus such as a digital camera.
2. Description of the Related Art
Conventionally, a phase difference detection method is generally known well as the auto focus detection method of a camera. In the phase difference detection method, light beams from an object, which have passed through different exit pupil areas of an imaging lens, form images on a pair of line sensors of an AF sensor. The relative positions of a pair of object images obtained by causing the pair of line sensors to photo-electrically convert the light are calculated (to be referred to as phase difference calculation hereinafter), thereby detecting the defocus amount of the imaging lens.
For example, Japanese Patent Laid-Open No. 2003-222786 discloses an auto focus solid-state image capturing apparatus of prior art. FIG. 13 is a block diagram showing the auto focus solid-state image capturing apparatus described in Japanese Patent Laid-accumulation time control based on the maximum value signal from an AF circuit 1101. A gain variable signal amplification circuit 1106 performs gain control based on the difference signal between the maximum value signal and the minimum value signal. The signal amplification circuit 1106 outputs a signal based on the minimum value signal. The dynamic range of the sensor is improved by the accumulation time control and gain control.
In the technique disclosed in Japanese Patent Laid-Open No. 2003-222786, however, gain control by the signal amplification circuit is performed at the preceding stage of the output terminal of the auto focus solid-state image capturing apparatus. If the S/N ratio before the signal amplification circuit is poor, it cannot be improved.
When the sensor has a low sensitivity, and the object has a very low brightness, the accumulation time needs to take longer to accurately calculate phase difference. At this time, noise generated by a dark current or in the circuit is nonnegligibly large relative to the accumulation signal so no satisfactory S/N ratio can be obtained. On the other hand, when the sensor has a high sensitivity, and the object has a very high brightness, the sensor is greatly affected by optical shot noise so no satisfactory S/N ratio can be obtained. In the AF sensor circuit, the signal amplification circuit amplifies an accumulation signal including noise. Hence, noise is amplified together, resulting in output with poorer S/N ratio.